If you look at the dashboard fuse box and trace the earth wiring back, it normally leads to one connection point on the vehicle, not far from the dashboard fuses (normally a bolt). If not, just use a bolt that is suitable for your earth wire nearby. I doubt your circuit will be pulling many amps.
I've used the above for approx. 15amp draw off subwoofers etc without issue for about 15 years now on various vehicles. The graphic suggest fuses need to be the same amps, but in practice I have violated this a fair few times and never had an issue come back at me. YMMV. It effectively makes a new circuit that are independent of each other in my experience.